# CRT Initialization#

## Default DLL entry point _DllMainCRTStartup#

In Windows, all DLLs can contain an optional entry-point function, usually called DllMain, that is called for both initialization and termination. This gives you an opportunity to allocate or release additional resources as needed. Windows calls the entry-point function in four situations: process attach, process detach, thread attach, and thread detach.

• process attach When a DLL is loaded into a process address space, either when an application that uses it is loaded, or when the application requests the DLL at runtime, the operating system creates a separate copy of the DLL data
• Thread detach occurs when the thread terminates, and process detach is when the DLL is no longer required and is released by an application.

The operating system makes a separate call to the DLL entry point for each of these events, passing a reason argument for each event type. For example, the OS sends DLL_PROCESS_ATTACH as the reason argument to signal process attach. The VCRuntime library provides an entry-point function called _DllMainCRTStartup to handle default initialization and termination operations

On process attach, the _DllMainCRTStartup function

• sets up buffer security checks
• initializes the CRT and other libraries
• initializes run-time type information
• initializes and calls constructors for static and non-local data
• initializes thread-local storage, increments an internal static counter for each attach
• and then calls a user- or library-supplied DllMain On process detach, the function goes through these steps in reverse.
• It calls DllMain
• decrements the internal counter
• calls destructors
• calls CRT termination functions and registered atexit functions
• notifies any other libraries of termination When the attachment counter goes to zero, the function returns FALSE to indicate to Windows that the DLL can be unloaded. The _DllMainCRTStartup function is also called during thread attach and thread detach. In these cases, the VCRuntime code does no additional initialization or termination on its own, and just calls DllMain to pass the message along If DllMain returns FALSE from process attach, signaling failure, _DllMainCRTStartup calls DllMain again and passes DLL_PROCESS_DETACH as the reason argument, then goes through the rest of the termination process.

## Reference#

Last update: 2022-09-07